1. Field of the Invention
The present invention generally relates to molecularly imprinted polymer ion exchange resins, a process for their preparation and their use as a sequestering agent for the removal of one or more inorganic ions from a liquid medium.
2. Description of the Related Art
Various levels of water purity may be required for different end uses. Water quality may be regulated by various government agencies and trade organizations including the U.S. Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA). Purified water is used in many industries including the chemical, foodstuffs, electronics, power, medical and pharmaceutical industries, as well as for human consumption. Typically, prior to use in any one of these fields, the water is treated to reduce the level of contaminants to acceptable levels. These treatment techniques include, for example, coagulation, disinfection, distillation, filtration, ion exchange, reverse osmosis, photooxidation, ozonation, and combinations thereof.
Disinfection units are typically used to reduce the concentration of viable microorganisms in a water supply. This may be accomplished by adding a disinfectant such as chlorine, ozone, hypochlorite, hypobromite or ammonia directly to the water supply so that pathogenic organisms are destroyed. Alternatively, microorganisms may be destroyed by a process, such as heating or treatment with ultraviolet light, or microorganisms may be physically removed from the water by filtration. When a chemical disinfectant is used, it is often desirable to remove the disinfectant from the water prior to consumption, and this may be accomplished in a number of ways including chemical neutralization and removal by filtration.
Filtration is used to remove suspended matter from a water supply but may also aid in the removal of dissolved or colloidal species. Filters may be structured from a variety of materials including particulate matter such as sand, diatomaceous earth, or granular activated carbon (GAC), or may be based on a membrane that may be composed of a number of different materials including polymers and fibrous materials. Filters typically work by preventing the passage of suspended material while allowing water to pass through. One way of rating a filter is by its “pore size” which provides information as to what size particle will be retained by the filter. Some methods, such as hyperfiltration, may have pore sizes small enough to exclude some dissolved species.
Water may be adversely affected by the presence of calcium or magnesium ions. Known as “hardness,” a high concentration of these cations, typically more than 200 ppm (mg/L as CaCO3), results in a water that may leave scale or other deposits on equipment and piping. Typically, calcium and magnesium are removed from water (softened) by exchanging the calcium and magnesium ions for alternative cations, often sodium. Water softeners typically contain resin beads that exchange two sodium ions for every calcium or magnesium ion that is removed from the treated water. Periodically, the water softener may be recharged to resupply the resin beads with an adequate supply of sodium or alternative cations.
Reverse osmosis (RO) is a filtration technique that provides for the removal of dissolved species from a water supply. Typically, water is supplied to one side of an RO membrane at elevated pressure and purified water is collected from the low pressure side of the membrane. The RO membrane is structured so that water may pass through the membrane while other compounds, for example, dissolved ionic species, are retained on the high pressure side. Some species however, such as bicarbonate, may not be retained. The “concentrate” that contains an elevated concentration of ionic species may then be discharged or recycled, while the permeate, typically containing a reduced concentration of ionic species, is collected for later use.
Deionization units may also be used to remove a variety of ionic species from a water supply. Deionization units typically employ either chemical or electrical deionization to replace specific cations and anions with alternative ions. In chemical deionization, an ion exchange resin is employed to replace ions contained in the feed water. The ions on the resin are recharged by periodically passing a recharging fluid through the resin bed. This fluid may be an acid that replenishes the supply of hydrogen ions on the cation exchange resin. For anion exchange resins, the resin may be replenished by passing a base through the resin, replacing any bound anions with hydroxyl groups and preparing the resin for additional anion removal.
The currently available technologies such as those discussed above require expensive equipment, large amounts of power, and are expensive to maintain. Other approaches, such as dilution with fresh water, are impractical over the long run.
Accordingly, there remains a need for systems and methods for removing contaminants from a liquid medium such as waste waters.